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Radiometric Dating - Overview
(Back to Radiometric Dating Home) Radiometric dating came about soon after the discovery of the structure of the atom. Over the past century it has grown and branched into many different forms, all sharing similar root concepts. The ideas of radiometric dating come directly from observations of radioactive decay. Radiometric dating has been applied to many different fields, though archeology and its connected disciplines are the most closely related. Radiometric dating does not necessarily need to be applied to the dating of specific artifacts, in many cases it has been used as a method to obtain information about the age of the Earth itself. __TOC__ History At the very end of the 19th century the term radioactivity was coined by Marie and Pierre Curie. They were the first to prove that radioactivity was a feature of atoms of an element not only molecules. This implied that it was an atomic property involving literal changes within the smallest level of matter as opposed to a standard chemical reaction or process. Within a few years of the Curies' discoveries, Ernest Rutherford and his associates were mapping radioactive decay patterns and establishing a system of classification and a framework for dating. In a 1905 lecture at Harvard, Rutherford suggested that measuring the ratios of radioactive isotopes and stable decay products could lead to the ages of materials. For the next half a century many scientists made attempts to assess the age of the Earth by radiometric means. However, there was a severe lack of understanding regarding the nature of isotopes and the subatomic mechanisms of radioactive decay. During World War II a large amount of research into the properties of radioactivity was conducted. This created very large advancements in methods of analyzing and understanding interactions at the subatomic level. After this, the field of radiometric dating truly began to grow.http://www.talkorigins.org/faqs/geohist.html#Radiometric From Theory to Method Once the basic theory of radioactive decay was understood, the applications to age determination were clear. It was observed by Rutherford that radioactive isotopes decay at an exponential rate. The rate of decay is related to the amount of the parent isotope that is present. For instance, suppose you have a 4g sample of Rubidium-87 (a radioactive isotope of the element Rubidium -- the stable isotope has a mass of 85). This sample will immediately begin to radioactively decay. As some of the sample releases beta particles and forms the decay product, the original sample will begin to decay more slowly. Scientists are able to map this decay rate mathematically and have discovered that exactly one half of the sample will decay in about 48 million years. This means that 2g of the original sample will remain at that point. After another half-life passes, only one gram of the original sample will remain. This rate is not constant but it is mathematically predictable. http://www.geo.cornell.edu/geology/classes/Geo656/656notes03/656%2003Lecture04.pdf This understanding of the mathematical nature of radioactive decay is what led to dating techniques. The main form of radiometric dating is geochronological methods. Methods in this category measure the ratio of parent and daughter isotopes (sometimes called radioactive and radiogenic species, respectively) and calculate the time that the sample has been decaying based on known half-life values. Samples can come in many forms, and often quantitative analytical methods are employed to measure very specific and accurate amounts of samples. While the basic method is the same in most geochronological cases, read on about specific types of dating. References #Stassen, C. (1998, June 9). Changing views of the history of earth. Retrieved from http://www.talkorigins.org/faqs/geohist.html#Radiometric #White, W. M. "Basics of Radioactive Isotope Geochemistry." Cornell University. 2003. http://www.geo.cornell.edu/geology/classes/Geo656/656notes03/656%2003Lecture04.pdf